Bisphenols as condensation products of phenols and carbonyl compounds are starting materials or intermediates for the production of a large number of commercial products. The condensation product from the reaction of phenol and acetone, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A, BPA), is of particular industrial importance. BPA is used as the starting material for the production of various polymeric materials, such as, for example, polyarylates, polyether imides, polysulfones and modified phenol-formaldehyde resins. Preferred fields of application are the production of epoxy resins and polycarbonates.
The industrially relevant preparation methods for BPA are based on the acid-catalyzed reaction of phenol with acetone, a phenol-acetone ratio of >5:1 preferably being established in the reaction. As acidic catalysts there may be used both homogeneous and heterogeneous Bronstedt or Lewis acids, for example strong mineral acids such as hydrochloric or sulfuric acid. Gel-like or macroporous sulfonated crosslinked polystyrene resins (acidic ion exchangers) are preferably used. The embodiments hereinbelow relate to a process using acidic ion exchangers as catalysts. These may be mono- or hetero-disperse.
In order to achieve high selectivities, the reaction of phenol with acetone is carried out in the presence of suitable mercapto compounds as co-catalysts. These may either be homogeneously dissolved in the reaction solution or fixed to the sulfonated polystyrene matrix via ionic or covalent bonds. The reaction unit is a stratified bed or fluidized bed, through which the solution flows upwards or downwards, or a column such as a reactive distillation column.
The selectivity of the reaction, as well as the long-term storage stability of the catalyst, are determined by the quality of the raw materials phenol and acetone that are used. For the preparation in particular of BPA as a raw material for high-quality plastics such as, for example, polycarbonate, very high demands are therefore made of the purity of the basic materials phenol and acetone that are used. Typically, purities of >99.95 wt. % for phenol and >99.90 wt. % for acetone, with at the same time low contents of impurities (S<0.5 ppm, Fe<1 ppm), are regarded as being positive for the achievement of high product purities and for minimizing catalyst deactivation. For example, EP-A-876 319 describes freeing commercial phenol of troublesome impurities by treatment with molecular sieve and thus ensuring better usability in a process for BPA preparation.
EP-A-680 913 describes the use of modified acidic ion exchangers to remove hydroxyacetone from phenol for BPA synthesis.
The reaction of phenol with acetone in the presence of acidic catalysts and mercapto compounds as co-catalysts yields a product mixture that contains unreacted phenol and optionally acetone and primarily BPA and water. In addition, small amounts of typical side products of the condensation reaction are formed, for example 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane (o,p-BPA), substituted indanes, hydroxyphenylindanols, hydroxyphenyl-chromanes, substituted xanthenes and more highly condensed compounds having three or more phenyl rings in the molecular framework.
The mentioned side products, as well as water, phenol and acetone, impair the suitability of BPA for the production of polymers and must be separated off by suitable processes. For the production of polycarbonate in particular, high demands are made of the purity of the raw material BPA.
The working up and purification of BPA is carried out by a multi-stage cascade of Suitable purification processes such as, for example, suspension crystallization, melt crystallization, distillation and/or desorption. In an industrially preferred form, BPA is separated from the reaction mixture in the form of an approximately equimolar crystalline adduct with phenol by cooling the reaction mixture with crystallization of the BPA/phenol adduct. The crystallization is preferably carried out in the form of suspension crystallization. The BPA/phenol adduct crystals are subsequently separated from the liquid phase by means of a suitable apparatus for solid/liquid separation, such as rotary filters or centrifuges, and fed to further purification. Adduct crystals so obtained typically have a purity of >99 wt. % BPA, based on BPA and the side products with a phenol content of about 40 wt. %, based on the total amount of adduct crystals. By washing with suitable solutions, which typically contain one or more components from the group acetone, water, phenol, BPA and side products, the adduct crystals may be freed of impurities adhering to the surface. The BPA/phenol adduct crystals obtained following the above-described suspension crystallization of the reaction solution and solid/liquid separation are fed to more extensive purification steps, wherein phenol is separated off and, optionally, a reduction in the concentration of side products is achieved by the use of suitable purification operations (suspension crystallization, layer crystallization, extraction, distillation).
The stream of liquid obtained in the solid/liquid separation (mother liquor) contains phenol, BPA, water formed in the reaction, unreacted acetone, and is rich in the side products typically formed in the preparation of BPA. In a preferred form, this stream of mother liquor is fed back into the reaction unit. In order to maintain the catalytic activity of the acidic ion exchanger, all or some of the water that has formed is removed beforehand by distillation, any acetone that is still present is also removed wholly or partially from the mother liquor. The dewatered reaction stream so obtained is supplemented with phenol and acetone and fed back into the reaction unit. Alternatively, it is possible to remove water and acetone wholly or partially by distillation before the suspension crystallization of the BPA/phenol adduct is carried out. In the mentioned distillation steps, a portion of the phenol present in the reaction solution may also be separated off by distillation.
Such a closed-circuit procedure has the disadvantage that side products of the BPA preparation become concentrated in the circulating stream; these side products adversely affect the purity of BPA in the suspension crystallization and may lead to deactivation of the catalyst system. In order to avoid excessive concentration of side products in the circulating stream, a portion of the mother liquor mixture must be discharged from the system. The amount of side products removed from the process in this manner must correspond in the equilibrium state to the amount of side products formed in the reaction. The discharge is typically effected by removing a portion of the mother liquor from the circulating stream, it being possible for water of reaction that has formed, unreacted acetone and portions of phenol optionally to be removed by distillation beforehand. The composition of the mother liquor at this point, and accordingly also the composition of the discharged portion, typically consists of from 70 to 90 wt. % phenol, from 3 to 15 wt. % BPA and from 3 to 15 wt. % side products and isomers formed in the reaction. Because ultimately only the last-mentioned portion of side products has to be removed from the process, the discharged amount is subjected to further working-up steps in order to minimize losses of material.
In a simple form, phenol is distilled off to a residual content of <10 wt. %, so that a residual resin having a content of <10 wt. % phenol, from 15 to 85 wt. % BPA and from 15 to 85 wt. % side products is obtained, which resin is removed from the process and disposed of, for example, by burning or dumping.
In another form, a portion of the BPA contained in the discharged amount is recovered by distilling off a portion of the phenol from the discharged portion and feeding the concentrated solution so obtained to suspension crystallization and subsequent solid/liquid separation. It has proved advantageous here to pass the discharged amount, before or after the partial separation of phenol, over a rearrangement unit filled with acidic ion exchanger. This unit is generally operated at higher temperatures than the reaction unit. In this rearrangement unit, under the conditions prevailing therein, some of the side products of the BPA preparation present in the circulating stream are isomerized to BPA, so that the total yield of BPA may be increased. In the solid/liquid separation, a portion of the BPA that is present is obtained in the form of a BPA/phenol adduct crystal and may be fed to further purification steps. A filtrate typically consisting of from 60 to 90 wt. % phenol, from 3 to 12 wt. % BPA and from 3 to 18 wt. % side products is additionally obtained. The phenol contained in this filtrate is distilled off to a residual content of typically <10 wt. % and the resulting residual resin, containing <10 wt. % phenol, from 14 to 80 wt. % BPA, from 20 to 86 wt. % side products, is fed to disposal.
The described processes for the working up of the discharged stream have the disadvantage that relatively large amounts of phenol, either in undissolved form or bonded in BPA or the side products, are still present in the residual resin that is finally disposed of Disposal of the residual resin accordingly leads to a loss of raw materials.
The object of the present invention was therefore to find a process for the working Lip of discharged streams from a BPA production process, with which process phenol is obtained in high purity and in high yields.
Such a process preferably meets the following demands:    1. minimization of phenol in the residual resin,    2. minimization of BPA in the residual resin,    3. minimization of the amount of residual resin,    4. provision of phenol of high purity (>99.8%) and having low contents of impurities (S, Fe, Cl) from the working-up process with high yields,    5. continuous procedure with minimal use of apparatus and energy.